During the past three decades, spacecraft power requirements have increased steadily from a few watts to multiple kilowatts as the complexity and sophistication of on-board equipment has grown. With this trend, the survivability and efficiency of the spacecraft's power generating system have become increasingly important.
As used herein, the term "spacecraft" includes both manned and unmanned spacecraft, as well as satellites. The power generating systems typically employed in spacecraft are nuclear or solar powered. Nuclear powered systems have been the subject of increasing criticism owing to the perceived danger of radioactive fall-out triggered by an accident or by strategic attack. Although solar power-generating systems are not subject to this type of mishap, they are vulnerable to degradation and destruction by both man-made and naturally occurring radiation, space plasma, high energy particles, space debris and kinetic energy particles.
Solar power-generating systems have typically included massive arrays of photocells which produce electrical energy when impinged upon by solar radiation. However, these photocells have been susceptible to damage from numerous sources in the hostile environment of space. In terms of natural phenomena, photocells are degraded by the impact of high energy ionic, atomic and molecular particles, as well as by electrons and neutral subatomic particles. Such high energy particles are especially abundant in the Van Allen Belt surrounding the earth. The effect of these impacts is a continuing degradation of photocell efficiency which ultimately renders the cell inoperable.
Current efforts to make photocell arrays less vulnerable to the aforedescribed threats include the hardening of the array by encapsulating the photocells within a thick glass shield. While shields and the like can help protect the photocells from damage due to collision with particulate matter, the extra weight which must be lofted that is attributable to the shield adversely effects the economics of the lift-off.
For the same economic reasons, an important measure of system performance is the number of "kilowatts per kilogram". However, because the efficiency of solar power-generating systems typically diminishes within a few years after launch owing to the performance-degrading effects of incident radiation and impacting particles on the photocells, solar power systems must be compensatorily overdesigned by as much as 30% so that energy production degrades to a still-acceptable limit. The consequently increased size and/or number of photocell arrays and other system components adversely adds to the weight which must be lofted.
Another obstacle to the survivability of high power on-board systems is the presence of space plasma which can short circuit and destroy the photocells that operate at high voltages. Although the theoretical vacuum of space is a perfect insulator, space plasma which is abundant in low level earth orbit electrically short circuits photocells. The plasma is characterized by a breakdown voltage that can be exceeded by the operating voltages of photocells in the high power-generating systems now needed by spacecraft. When the breakdown voltage is exceeded, the plasma conducts an electrical current, short-circuiting the photocell and rendering it inoperative.
Turning to man-made photocell-degrading phenomenon, x-rays and gamma rays, such as those generated by nuclear explosions, as well as high power laser beams, directed energy beams and kinetic energy projectiles can all be used to degrade and/or destroy the photocell array. When any one photocell in an array is destroyed, a number of other cells in the array are often effectively rendered inoperable because the cells are electrically connected in series. Since each photocell produces a maximum of approximately 1 volt, the series arrangement is used to boost the generated voltage up to the 28-100 volts utilized by the on-board equipment. As a result, the power-generating system of the spacecraft can quickly become inadequate or inoperative when a limited number of the photocells are destroyed, effectively knocking out a greater number of cells.